Assume you have the following 2 classes:
class Entity<IDT,NameT>{
    IDT id;
    NameT name;

    public Entity(IDT id, NameT name) {
        super();
        this.id = id;
        this.name = name;
    }
    public IDT getId() {
        return id;
    }
    public NameT getName() {
        return name;
    }
}

class Employee extends Entity<Integer,String>{
    double age; 
    private int tag;

    public Employee() {
        super(0,null);
    }
    public Employee(int id, String name, double age) {
        super(id,name);
        this.age = age;
    }

    public double getAge() {
        return age;
    }

    void setTag(int tag){
        this.tag = tag;
    }   
}

when trying to use 
  new GsonBuilder().create().toJson(new Employee(1,"Mary",10)); it throws 

java.lang.UnsupportedOperationException: Expecting parameterized type, got
class test.gson.Employee.
 Are you missing the use of TypeToken idiom?
 See
http://sites.google.com/site/gson/gson-user-guide#TOC-Serializing-and-Deserializ
ing-Gener

ok, then I try the generic one using 
new GsonBuilder().create().toJson(new Employee(1,"Mary",10),new
TypeToken<Employee>(){}.getType()); Gson will only show id/name field, all
other field in Employee class is ignored. any solution? 

I've refered <a
href="http://code.google.com/p/google-gson/source/browse/trunk/gson/src/test/jav
a/com/google/gson/functional/ParameterizedTypesTest.java">,
but didn't help.

please help to solve the problem. thanks!